In order to provide protection against power outages, modern computer systems typically use battery backups. When the AC voltage supplied to the computer drops below a certain voltage level, a trigger circuit will cause a battery voltage to be discharged to the computer. This allows the computer to save its memory during the outage, and if necessary, to perform a graceful shutdown.
Typically, the battery voltage is supplied by a plurality of battery packs, each containing a plurality of battery cells. These batteries are coupled in series to provide the input voltage when the normal input source fails. The battery packs must be kept charged to assure that the battery will be able to supply the proper level of voltage to the computer, when called upon to do so. This function is performed by a battery charger.
In the prior art, battery chargers are known which sense the voltages of the series-coupled battery packs, and then provide a charge to simultaneously raise the voltage levels of each battery pack. The charge will be applied until the voltage of the lowest voltage pack is raised beyond a predetermined threshold voltage level. The problem with this approach is that the battery packs do not often have the same voltage levels before charging. Therefore, when the lowest voltage battery pack is raised to the predetermined threshold level, the other battery packs are potentially charged to much higher voltage levels. In other words, in order to assure that the lowest voltage battery has a minimum voltage level, the prior art battery chargers allow the possibility of the remaining battery packs being charged to too high a level.
There is therefore a need to provide a battery charger that would keep a plurality of battery packs charged to a minimum voltage level, but at the same time assure that these 3-battery packs are each charged to the same voltage levels.